[Crash-utility] Re: backtrace - how do I get local and formal parametes - kernel is compiled -O0.

[Pete/Piet Delaney]

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Dave Anderson wrote:
| Pete/Piet Delaney wrote:
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|  >
|  > Dave Anderson wrote:
|  > | Pete/Piet Delaney wrote:
|  > |> Got by 1st panic crash, crash seems to be working but
|  > |> I don't feel comfortable as with gdb. I can't see
|  > |> the parameters on the stack as variables as with
|  > |> kgdb nor can I see the local variables on the stack.
|  > |> Seems I should be able to get crash working with
|  > |> gdb so I can see the details that gdb can see.
|  > |>
|  > |> I compiled the kernel -O0 and without the framepointer
|  > |> as it seem crash prefers (rather odd). But want to
|  > |> see the locals and formals on the stack and walk up and
|  > |> down the stack and have the context know to crash just
|  > |> as with gdb.
|  > |>
|  > |> I'll re-read your crash paper.
|  > |>
|  > |> Any suggestions?
|  > |
|  > | Nope, other than using "bt -f" to dump each frame's data,
|  > | and figuring it out from there...
|  >
|  > When I was using gdb on SunOS 4.1.4 I was able to set $sp and
|  > $fp to switch processes. When I took the SP and FP and used
|  > a gdb set $sp and %fp it didn't seen to allow me to do this:
|  > - -----------------------------------------------------------
|  > crash> gdb set $sp = 0xd76bbbb4
|  > No registers.
|  >
|  > crash> gdb set $fp = 0xd76bbc24
|  > No registers.
|  > - -----------------------------------------------------------
|  > Why is is saying 'No registers'?
|
| I'm not positive, but the gdb sources seem to display that message
| whenever (!target_has_registers), and that's #define'd like so:
|
|   /* Does the target have registers?  (Exec files don't.)  */
|
|   #define target_has_registers    \
|        (current_target.to_has_registers)
|
| And since it's being invoked as "gdb vmlinux", it only knows
| about the exec file, and has no clue about registers.

Perhaps gdb changed since SunOS 4.1.4 in this area. We could
consider patching gdb to think it has registers just to allow
crash and/or the user from setting $sp and $fp to facilitate
better back trace information. As I recall you make environment
isn't building a virgin gdb.



|  >
|  > Also why don't you set up gdb with the stack info for the current
|  > task and either allow the user to do a 'gdb bt' or do it with a
|  > crash 'bt' option?
|  >
|
| I don't even come close to understanding gdb internals well enough
| to know what "set up gdb with the stack info" entails.  Send me
| a patch when you've got it working -- for all arches, for all tasks,
| and without requiring any vmcore file argument...  ;-)
|
|  > Will gdb work directly with the kdump core file? If not when did
|  > it break and why? Docs with 2.6.16 suggest to use gdb of the core
|  > file.
|
| I don't know if it *has* broke...
|
| As I said this morning, I don't know if the latest and greatest 32-bit
| gdb can work with 64-bit ELF kdump vmcores, which kdump uses by default
| for both 32-bit and 64-bit architectures.  The kdump.txt file says this:
|
|   * By default, the ELF headers are stored in ELF64 format to support
|     systems with more than 4GB memory. The --elf32-core-headers option can
|     be used to force the generation of ELF32 headers. This is necessary
|     because GDB currently cannot open vmcore files with ELF64 headers on
|     32-bit systems. ELF32 headers can be used on non-PAE systems (that is,
|     less than 4GB of memory).
|
| But at least with a 32-bit ELF vmcore, gdb should be able to work.

ok, I'll trying pulling out a DIMM and running on 2GB.

|
|  >
|  > I recall the crash backtrack proving the gdb info back a few years
|  > ago when I worked on LKCD and crash with you. Is my memory wrong
|  > and this visibility of the formals and local variables has never
|  > never been possible? It's hard to recall for sure with my using
|  > kgdb on the kernel now for so long.
|  >
|
| Your memory's wrong, especially w/respect to local variables -- are
| you kidding me?

No, I wasn't joking, I thought back traces with crash had the same
format as with gdb. Last I worked on IA64 and gdb I found messing
with it's accessing of registers rather convolved. I was modifying
Andy Kleen's gdb interface to SGI's kdb debugger, it's called 'SKDB'.
At the time SGI had kdb working on IA64 kernels and I tried modifying
if from ia32 to IA64. The register storage in gdb was a bit odd.

Seems to be worthy of consideration to fool gdb into thing it has
registers just so crash can get gdb to provide backtrace info.
What happens of you simply change the #define to something
like:

	#define target_has_registers	1

I take it that crash in doing the stack unwind it's self now
instead of having gdb do it. Have you thought about having gdb
do it and what's needed?

On Friday you mentioned an embedded  gdb module to hijack read
attempts and back them to crash. I tried a Google for more info
but didn't find anything. Could you tell me more about that?

- -piet

|
| Dave
|
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